This invention relates generally to systems for generating high-frequency fields, and more particularly, to a system for generating a substantially homogeneous, high-frequency magnetic field for use in a nuclear magnetic resonance apparatus.
In nuclear magnetic resonance systems, particularly for nuclear spin resonance tomography or zeugmatography, high-frequency magnet systems are used for producing high-frequency magnetic fields and for receiving high-frequency signals. A known arrangement for practicing zeugmatography is provided with at least two conductor sections of predetermined length which extend along at least one imaginary cylindrical surface in a direction parallel to a cylindrical axis. Current flows through the two conductor sections in opposite directions; the conductor sections being connected to at least one external power supply or signal receiver, respectively. A system of this type is described in European patent application EP 21 535 Al.
In the field of medical diagnostics, imaging methods have been proposed in which an image which is similar to an X-ray tomogram is constructed by the computed or measured analysis of integrated proton resonsance signals from the spatial spin density and/or relaxation time distribution of a body to be examined. This method, which is also called "zeugmatography", is described in the publication "Nature", volume 242, 1973, pages 190-191.
During the application of nuclear spin resonance, the body to be examined, which may be a human body, is placed in a strong homogeneous magnetic field. The magnetic field, which is also called the base field, is superimposed by constant and/or pulsed gradient fields. Moreover, a high-frequency field which is oriented perpendicularly in the base field, must be developed which is also as homogeneous as possible. The physical dimensions of the system must be matched to the dimensions of the body to be examined so that the body can be placed, without difficulty, into the conductor system which generates the magnetic fields.
It is well known that in the apparatus of nuclear spin tomography or zeugmatography, the quality of sectional images which are produced depends upon the signal-to-noise ratio of the induced nuclear spin resonance signal. Since the signal-to-noise ratio depends upon the magnetic base field and its frequency, it is desirable to provide frequencies as high as possible in the base fields. See, for example, "J. Phys. E. Sci. Instrum.", volume 13, 1980, pages 38-44.
In the known nuclear magnetic resonance systems, only relatively low frequencies of approximately 5 MHz can be provided as a result of physical limitations. The high-frequency magnet fields are produced by coils which have the shape of a saddle. Such coils, which are arranged on at least one imaginary cylindrical surface, have straight conductor sections which extend parallel to the direction of the axis of the cylinder, and are provided at the end faces with arc-shaped conductor sections which extend in the circumferential direction. In this manner, the current in the conductor section in one straight side of the coil flows in a direction which is opposite to the flow of current in the other straight side. The axial length of the straight conductor sections and therefore, of the coils, is predetermined by the dimensions of the body to be examined. The entire wire length of the coil may be, for example, on the order of several meters. In coils of such dimensions, however, phase shifts of the fields between the beginning and end of the conductor of the coils occurs at the desired higher frequencies, above 5 MHz. This occurs because the lengths of the conductor are in the order of magnitude of the wavelength of the high frequency. As a result of these phase shifts, the quality of the fields which are generated by the coils decreases with increasing frequency. Moreover, undesirable coupling occurs with metallic parts of the component which surround the high frequency coils, such as the gradient coils or the base field magnet. This produces additional disturbances and eddy currents which attenuate the high-frequency field in the interior of the body to be examined. Moreover, the known "skin effect" renders the field in the body to be examined inhomogenenous. It is, therefore, not directly possible to increase the frequencies of operation and thereby improve the signal-to-noise ratio of the known nuclear magnetic resonance equipment.
It is, therefore, an object of this invention to provide a nuclear magnetic resonance apparatus having an arrangement for generating an essentially homogeneous magnetic high-frequency field in such a manner that a relatively large signal-to-nosie ratio is achieved which would correspond to an operating frequency of illustratively 20 MHz.